1. Field of the Invention
The invention relates to scanning probe microscopes (SPMs). More particularly, it is directed to a method and apparatus for monitoring movement of an atomic force microscope (AFM) actuator using an auxiliary position sensor.
2. Discussion of the Prior Art
A scanning probe microscope operates by providing relative scanning movement between a measuring probe and a sample while measuring one or more properties of the sample.
Commonly, an actuator such as a piezoelectric tube (often referred to herein as a “piezo tube”) or flexure is used to generate relative motion between the measuring probe and the sample surface. A piezoelectric tube is a device that moves in one or more directions when voltages are applied to electrodes disposed inside and outside the tube. Actuators may be combined in an arrangement coupled to the probe, the sample, or both. Most typically, an actuator arrangement includes an X-Y actuator that drives the probe or sample in a horizontal, or XY plane and a Z actuator that moves the probe or sample in a vertical, or Z direction.
Piezoelectric tubes and other types of actuators are imperfect. For example, a piezo tube often does not move only in the intended direction. When a Z actuator, for instance, is commanded to move in the Z-direction by the application of an appropriate voltage between the actuator's electrodes, the Z actuator moves not only in the Z direction, but in the X and/or Y directions as well. This unwanted parasitic motion limits the accuracy of measurements obtained by scanning probe microscopes. The amount of this parasitic motion varies with the geometry of the tube and with the uniformity of the tube material, but typically cannot be eliminated to achieve the accuracy required by present instruments.
Methods of monitoring the motion of the probe or sample when driven by a SPM actuator have been developed in an attempt to compensate for this parasitic X and Y error, with mixed results. The devices are typically calibrated by applying a voltage to the X-Y actuator and the Z actuator, and then measuring the actual distance that the sample or probe travels. Thus, the position of the actuator is estimated by the voltage that is applied to the X-Y actuator and the Z actuator. However, correcting for the (X,Y) position error introduced by the Z actuator on the probe or sample is difficult because it requires additional calibration steps and more complex circuitry to determine the correct voltage to apply to the X-Y actuator.
Moreover, with respect to movement in the intended direction, piezoelectric tubes and other types of actuators typically do not move in a predictable way when known voltages are applied. The ideal behavior would be that the actuator be “linear,” i.e., move in exact proportion to the voltage applied. Instead actuators, including piezo tubes, often move in a non-linear manner, meaning that their sensitivity (e.g., nanometers of motion per applied voltage) can vary with variations in voltage. In addition, they suffer from hysteresis and creep. Most generally, the response to an incremental voltage change will depend on the history of previous voltages applied to the actuator. These effects can cause a large prior motion to affect the response of a commanded move, even minutes later.
Similarly, vertical measurements in scanning probe microscopy are typically calculated mathematically by recording the voltage applied to the Z actuator and then multiplying by the actuator's calibrated sensitivity in nm/V. However, as mentioned previously, this sensitivity is not constant and most often depends on the previous voltages applied to the actuator. Using the voltage applied to the actuator to calculate the vertical motion of the actuator therefore will always result in an error with respect to the actual motion. This error can translate directly into errors when measuring surface topography of a sample and performing other metrology experiments. These issues have been addressed specifically for the case in which the probe assembly of the AFM is coupled to the actuator (i.e., the case in which the probe assembly moves in three orthogonal directions referenced below).
Some SPMs attempt to improve accuracy by using an auxiliary displacement sensor to actively monitor actuator movement and adjusting the voltage to the appropriate SPM actuator(s) to cause the actuator to move in the desired manner. Several auxiliary displacement sensors, sometimes referred to as position sensors, have been proposed for monitoring actuator movement, including Linear Variable Displacement Transducers (LVDTs), capacitance sensors, strain gauge sensors, and optical displacement sensors (ODSs). ODSs, sometimes referred to as optical position sensors or OPSs, are preferred in many applications because they are sufficiently compact to fit in a SPM scan head and exhibit lower power dissipation than many other known sensors. It is with ODSs that the invention is concerned.
One ODS was installed on a SPM marketed by Park Scientific Instruments and generally disclosed, for example, in U.S. Pat. No. 5,376,790 to Linker et al. (the Linker patent). Referring to FIG. 1A, the commercial version of the ODS disclosed in the Linker patent includes a fixed light source and a photodetector that moves with the SPM actuator. In that device, a tube scanner 700 is mounted in x,y coarse stage 116 of the SPM. Mounted at the base of tube scanner 700 is a light emitting diode (LED) 800, which directs a light beam 802 upward along the vertical axis of tube scanner 700. A sample platform (not shown) is mounted at the top of tube scanner 700. A quad-cell position-sensitive photodetector (PSPD) 808 is mounted in line with the axis of tube scanner 700. A slit aperture 810 is positioned between the LED 800 and the photodetector 808 to reduce shot noise. In use, movement of the tube scanner in the X or Y direction results in corresponding movement of the LED 800, which is detected by the photodetector 808.
The Park Scientific ODS works reasonably well but experiences drawbacks. For instance, it has very low sensitivity because the light reaching the photodetector 808 has relatively low intensity. Noise of about 5-10 nm RMS with a bandwidth of about 1 kHz is typical. The Park Scientific ODS also requires an electrical connection to a moving object, in this case the quad cell, adding to the complexity and weight of the system. It also increases the risk of electrical interference with the photodetection.
Another type of ODS is described in U.S. Pat. Nos. 6,612,160, 6,530,268, and 6,920,863 to Massie (the Massie patents) assigned to Veeco and commercialized in the Hybrid XYZ scanner. One of the arrangements disclosed in the Massie patents is shown in FIG. 1B. In that device, an ODS 900 is used to monitor displacement of an actuator arrangement 902. The ODS 900 includes a light source 906 that is fixed relative to the actuator arrangement 902, a photodetector 908 in the form of a position sensitive linear photodiode that is also fixed relative to the actuator arrangement 902, and a lens 910 that is positioned between the light source 906 and the photodetector 908 and that moves with the actuator arrangement. The lens 910 magnifies the light projected onto the photodetector 908 by a factor of 1+i/o, where “i” is the orthogonal distance from sensor 908 to lens 910, and “o” is the orthogonal distance from lens 910 to light source 906. In operation, as actuator arrangement 902 is actuated and caused to move in a particular direction, lens 910 correspondingly moves, thereby causing the location of light impingement onto the photodetector 908 to move. Because both the light source 906 and the photodetector 908 are fixed, measuring the position at which the light beam “L” output by lens 910 impinges on photodetector 908 is indicative of the movement of the actuator arrangement 902. The lens 910 magnifies the motion of the light on the detector during this process in an effort to increase sensitivity.
This ODS 900 has the advantage of lacking electrical connections to a moving object. However, the magnifying lens also increases the size of the spot on the photodetector. This requires that a larger photodetector be used to maintain linearity, which in turn results in a proportional decrease in sensitivity and signal to noise ratio. This effect can be alleviated by the use of a light source with exceptionally small emitting area, but such sources are expensive and have high power dissipation.
The need therefore has arisen to provide an ODS for a SPM that has very high sensitivity and low cost.
The need has additionally arisen to provide an optically based method of monitoring movement of a SPM actuator.